In U.S. Pat. Nos. 5,844,086 and 6,005,076 (“Murray II”), assigned to the assignee hereof and the disclosures of which are incorporated herein by reference, there is described a process for the isolation of protein isolates from oil seed meal having a significant fat content, including canola oil seed meal having such content. The steps involved in this process include solubilizing proteinaceous material from oil seed meal, which also solubilizes fat in the meal, and removing fat from the resulting aqueous protein solution. The aqueous protein solution may be separated from the residual oil seed meal before or after the fat removal step. The defatted protein solution then is concentrated to increase the protein concentration while maintaining the ionic strength substantially constant, after which the concentrated protein solution may be subjected to a further fat removal step. The concentrated protein solution then is diluted to cause the formation of a cloud-like mass of highly associated protein molecules as discrete protein droplets in micellar form. The protein micelles are allowed to settle to form an aggregated, coalesced, dense amorphous, sticky gluten-like protein isolate mass, termed “protein micellar mass” or PMM, which is separated from residual aqueous phase and dried.
The protein isolate has a protein content (as determined by Kjeldahl nitrogen or other convenient procedure N×6.25) of at least about 90 wt %, is substantially undenatured (as determined by differential scanning calorimetry) and has a low residual fat content. The term “protein content” as used herein refers to the quantity of protein in the protein isolate expressed on a dry weight basis. The yield of protein isolate obtained using this procedure, in terms of the proportion of protein extracted from the oil seed meal which is recovered as dried protein isolate was generally less than 40 wt %, typically around 20 wt %.
The procedure described in the aforementioned Murray II patent was developed as a modification to and improvement on the procedure for forming a protein isolate from a variety of protein source materials, including oil seeds, as described in U.S. Pat. No. 4,208,323 (Murray IB). The oil seed meals available in 1980, when U.S. Pat. No. 4,208,323 issued, did not have the fat contamination levels of the canola oil seed meals available at the time of the Murray II patents, and, as a consequence, the procedure of the Murray IB patent cannot produce from such oil seed meals, proteinaceous materials which have more than 90 wt % protein content. There is no description of any specific experiments in the Murray IB patent carried out using rapeseed (canola) meal as the starting material.
The Murray IB patent, itself was designed to be an improvement on the process described in U.S. Pat. Nos. 4,169,090 and 4,285,862 (Murray IA) by the introduction of the concentration step prior to dilution to form the PMM. The Murray IA patents describe one experiment involving rapeseed but provides no indication of the purity of the product. The concentration step described in the Murray IB patent served to improve the yield of protein isolate from around 20% for the Murray IA process.
One difficulty which the canola protein isolates produced by such prior procedures possess is a relatively dark yellow colour and an undesirable flavour. Phenolic compounds have been reported to be responsible for these problems of canola protein products including meal. Canola contains about ten times the quantity of phenolic compounds as is found in soybeans and may comprise sinapine and condensed tannins. Upon oxidation, phenolic compounds can give rise to the development of a dark colour. This problem is particularly acute with canola protein products produced by isoelectric precipitation where the strongly alkaline conditions lead to ready oxidation of phenolic compounds to quinones, which then react with the protein and impart a dark green or brown colour to the protein and solutions thereof. Other compounds and reactions also may contribute to colour formation.